KUFSTEIN, Austria, June 20 (Reuters) - Switzerland's Novartis pledged to nearly triple its number of biosimilar drugs on the market by 2020, raising its bet that cheaper versions of blockbuster cancer and immune system medicines will snatch billions in rivals' profits.

Novartis's Sandoz generics unit aims to be selling eight biosimilars, compared with three now, as patents on original drugs expire.

Sandoz head Richard Francis said his versions of AbbVie's $13 billion blockbuster Humira, Amgen's Enbrel and Neulasta, Johnson & Johnson's Remicade and Roche's Rituxan would arrive in pharmacies over the next four years, barring stumbles.

The five original drugs booked $44 billion in annual sales in 2015, combined.

Francis is wagering biotech copies that work like their forerunners despite slight variations - no biologic drug can be exactly duplicated - will be irresistible for insurers, health care systems and governments aiming to contain costs.

"We think we can become a real partner for payers and health care systems in making sure they can manage their budgets," Francis said in an interview at his new 150 million euro ($169 million) facility in Austria's Tyrolia region.

Again and again, governments are rebelling against the cost of medicines.

"Biosimilars are not something you dip your toe into," Francis said. "We're full on into it."

His boss, Novartis Chief Executive Joe Jimenez, predicts biosimilars may cost 75 percent less than the drugs they mimic, a deeper discount than originally expected.

What he loses on price, however, Jimenez aims to make up in volume.

According to consultancy IMS Health, cheaper biosimilars will save health systems in the United States and European Union between 50 billion and 100 billion euros by 2020 - especially as doctors overcome reservations about copies.

Convincing physicians remains a work in progress, said Sandoz biopharma development head Mark McCamish. When many U.S. doctors hear regulatory terms like "highly similar", he said, what they hear is "different".

'CONSTANT FIGHT'

"Those are terms we constantly have to fight," McCamish said.

Biosimilars arrived in Europe a decade ago, though their American introduction dragged until last year, when Novartis's Zarxio, a copy of Amgen's Neupogen drug to fight infection risk in cancer patients, became the first U.S. Food and Drug Administration (FDA) approval after a protracted court fight.

On Sandoz launches through 2020, biosimilars head Carol Lynch declined to give her specific launch schedule, or say which would be first. That is partly in regulators' hands, she said, with at least eight biosimilars including from other companies awaiting the FDA's green light.

"Right now, there is a delay for many files," Lynch said.

While the European Medicines Agency has approved more than 20 biosimilar drugs including three from Novartis since 2006, the FDA has approved just two: Zarxio last September and more recently, Celltrion's version of Remicade for autoimmune diseases.

Beyond regulatory hang-ups, Novartis's 2020 aspirations are also exposed to lingering legal hurdles, though the company is optimistic those will be resolved.

In one case, Novartis has petitioned the U.S. Supreme Court to reconsider a lower court ruling forcing biosimilar makers to wait six months after FDA approval before starting sales.

"We expect to hear about that over the course of the summer," Lynch said.

Original drugmakers are extending their elbows. As the main patent on arthritis treatment Humira, the world's best-selling drug, expires in December, AbbVie has warned imitators it believes additional patents protect it until 2022.